The evolution of inaccurate mimics

Instances of strikingly accurate Batesian mimicry (in which a palatable prey organism closely resembles an aversive model) are often cited to illustrate the power of natural selection. Less attention has been paid to those mimics, such as many hoverfly (Syrphidae) mimics of wasps or bees, that resemble their models only poorly. Attempts to provide an adaptive explanation for imperfect mimicry have suggested that what seems a crude resemblance to human observers may appear a close match to predators, or that inaccurate mimics may bear a general resemblance to several different model species. I show here, however, that truly inaccurate mimicry of a single model organism may be favoured over perfect resemblance, by kin selection. Signal detection theory predicts that predators will modify their level of discrimination adaptively in response to the relative frequencies and similarity of models and mimics. If models are rare and/or weakly aversive, greater local similarity of mimics can thus lead to greater attack rates. Where individual mimics are related to others in their vicinity, kin selection will then oppose the evolution of accurate mimicry.     

Co-mimics have a mutualistic relationship despite unequal defences

In the first clear mathematical treatment of natural selection, Müller proposed that a shared warning signal (mimicry) would benefit defended prey species by sharing out the per capita mortality incurred during predator education. Although mimicry is a mainstay of adaptationist thinking, there has been repeated debate on whether there is a mutualistic or a parasitic relationship between unequally defended co-mimic species. Here we show that the relationship between unequally defended species is mutualistic. We examined this in a 'novel world' of artificial prey with wild predators (great tit, Parus major). We kept the abundance of a highly defended prey ('model') constant and increased the density of a moderately defended prey ('defended mimic') of either perfect or imperfect mimetic resemblance to the model. Both model and defended mimic showed a net benefit from a density-dependent decrease in their per capita mortality. Even when the effect of dilution through density was controlled for, defended mimics did not induce additional attacks on the model, but we found selection for accurate signal mimicry. In comparison, the addition of fully edible (batesian) mimics did increase additional attacks on the model, but as a result of dilution this resulted in no overall increase in per capita mortality. By ignoring the effects of density, current theories may have overestimated the parasitic costs imposed by less defended mimics on highly defended models.     

The evolution of müllerian mimicry in multispecies communities

Prey species that are unprofitable to attack often share conspicuous colours and patterns with other coexisting defended species. This phenomenon, termed müllerian mimicry, has long been explained as a consequence of selection on defended prey to adopt a common way of advertising their unprofitability. However, studies using two unpalatable prey types have not always supported this theory. Here we show, using a system of humans hunting for computer-generated prey, that predators do not always generate strong selection for mimicry when there are two unprofitable prey types. By contrast, we demonstrate that when predators are faced with a range of different prey species, selection on unprofitable prey to resemble one another can be intense. Here the primary selective force is not one in which predators evaluate the profitabilities of distinct prey types independently, but one in which predators learn better to avoid unprofitable phenotypes that share traits distinguishing them from profitable prey. This need to simplify decision making readily facilitates the spread of imperfect mimetic forms from rarity, and suggests that müllerian mimicry is more likely to arise in multispecies communities.     

Selection overrides gene flow to break down maladaptive mimicry

Predators typically avoid dangerous species, and batesian mimicry evolves when a palatable species (the 'mimic') co-opts a warning signal from a dangerous species (the 'model') and thereby deceives its potential predators. Because predators would not be under selection to avoid the model and any of its look-alikes in areas where the model is absent (that is, allopatry), batesian mimics should occur only in sympatry with their model. However, contrary to this expectation, batesian mimics often occur in allopatry. Here we focus on one such example--a coral snake mimic. Using indirect DNA-based methods, we provide evidence suggesting that mimics migrate from sympatry, where mimicry is favoured, to allopatry, where it is disfavoured. Such gene flow is much stronger in nuclear genes than in maternally inherited mitochondrial genes, indicating that dispersal by males may explain the presence of mimetic phenotypes in allopatry. Despite this gene flow, however, individuals from allopatry resemble the model less than do individuals from sympatry. We show that this breakdown of mimicry probably reflects predator-mediated selection acting against individuals expressing the more conspicuous mimetic phenotype in allopatry. Thus, although gene flow may explain why batesian mimics occur in allopatry, natural selection may often override such gene flow and promote the evolution of non-mimetic phenotypes in such areas.     

Visual predators select for crypticity and polymorphism in virtual prey

Cryptically coloured animals commonly occur in several distinct pattern variants. Such phenotypic diversity may be promoted by frequency-dependent predation, in which more abundant variants are attacked disproportionately often, but the hypothesis has never been explicitly tested. Here we report the first controlled experiment on the effects of visual predators on prey crypticity and phenotypic variance, in which blue jays (Cyanocitta cristata) searched for digital moths on computer monitors. Moth phenotypes evolved via a genetic algorithm in which individuals detected by the jays were much less likely to reproduce. Jays often failed to detect atypical cryptic moths, confirming frequency-dependent selection and suggesting the use of searching images, which enhance the detection of common prey. Over successive generations, the moths evolved to become significantly harder to detect, and they showed significantly greater phenotypic variance than non-selected or frequency-independent selected controls.     

Ecosystem size determines food-chain length in lakes

Food-chain length is an important characteristic of ecological communities: it influences community structure, ecosystem functions and contaminant concentrations in top predators. Since Elton first noted that food-chain length was variable among natural systems, ecologists have considered many explanatory hypotheses, but few are supported by empirical evidence. Here we test three hypotheses that predict food-chain length to be determined by productivity alone (productivity hypothesis), ecosystem size alone (ecosystem-size hypothesis) or a combination of productivity and ecosystem size (productive-space hypothesis). The productivity and productive-space hypotheses propose that food-chain length should increase with increasing resource availability; however, the productivity hypothesis does not include ecosystem size as a determinant of resource availability. The ecosystem-size hypothesis is based on the relationship between ecosystem size and species diversity, habitat availability and habitat heterogeneity. We find that food-chain length increases with ecosystem size, but that the length of the food chain is not related to productivity. Our results support the hypothesis that ecosystem size, and not resource availability, determines food-chain length in these natural ecosystems.     

The effects of competition and predation on diversification in a model adaptive radiation

Much of life's diversity is thought to have arisen through successive rounds of adaptive radiation-the rapid diversification of a lineage into a range of ecologically and phenotypically distinct species. Both resource competition and predation have been suggested as mechanisms driving this process, although the former is better studied than the latter. Here we show experimentally how predation by a protist, Tetrahymena thermophila, affects diversification in a model adaptive radiation of the bacterial prey, Pseudomonas fluorescens. We estimate the frequency-dependent fitness functions of competing niche-specialist prey in the presence and absence of predation, and use these to test hypotheses about the extent (measured as the number of new genotypes) and rate of diversification. Competition and predation independently generated diversifying selection that we show is capable of driving prey diversification to similar extents but at different rates, diversification being markedly delayed in the presence of predators. The cause of this delay stems from weaker diversifying selection due to the reduction in prey density caused by predation. Our results suggest that predation may play an under-appreciated role in driving adaptive radiations.     

Release of invasive plants from fungal and viral pathogens

Invasive plant species both threaten native biodiversity and are economically costly, but only a few naturalized species become pests. Here we report broad, quantitative support for two long-standing hypotheses that explain why only some naturalized species have large impacts. The enemy release hypothesis argues that invaders' impacts result from reduced natural enemy attack. The biotic resistance hypothesis argues that interactions with native species, including natural enemies, limit invaders' impacts. We tested these hypotheses for viruses and for rust, smut and powdery mildew fungi that infect 473 plant species naturalized to the United States from Europe. On average, 84% fewer fungi and 24% fewer virus species infect each plant species in its naturalized range than in its native range. In addition, invasive plant species that are more completely released from pathogens are more widely reported as harmful invaders of both agricultural and natural ecosystems. Together, these results strongly support the enemy release hypothesis. Among noxious agricultural weeds, species accumulating more pathogens in their naturalized range are less widely noxious, supporting the biotic resistance hypothesis. Our results indicate that invasive plants' impacts may be a function of both release from and accumulation of natural enemies, including pathogens.     

陕西省紫柏山区食蚜蝇科昆虫的调查

通过对紫柏山区食蚜蝇科昆虫系统调查,结合整理陕西资源生物重点实验室馆藏标本,记录了紫柏山地区食蚜蝇科昆虫53种,隶属于2亚科、12族,其中中国新记录4种,陕西省新记录5 种．     

Coexistence of two sympatric and mOrpho1ogically similar bat species Rhinolophus affnis and Rhinolophus pearsoni

Ecologists have long focused on the coexistence of sympatric species.Here,we investigated two horseshoe bat species,Rhinolophus affinis and Rhinolophus pearsoni inhabited in the same cave,for their foraging strategies,niche differentiation,prey selection,and their coexistence status.These two species of horseshoe bats were ditierent in the dominant frequency of their echolocation calls.but similar in their morphology.We found evidence for prey selectivity although there was a high degree of overlap in prey categories and sizes.R affinis and R pearsoni foraged on 16 and 7 categories insects,respectively,with Pyralidae,Geometridae,Melolonthidae dominating their diets.The degree of trophic niche overlap was 0.69.Pairwise comparisons suggested that there was no obvious differentiation in prey categories and size.However,high prey availability in the environment(Simpson diversity index=0.79 and Margalef richness index=4.121 contributed to their coexistence by dampening the interspecific competition.Since there are one or more mechanisms facilitating species coexistence in a community,our results suggest that the spatial niche differentiation in foraging microhabitats and in foraging habitats at landscape scale may promote the coexistence of the two bat species.However,additional field data are needed to confirm this speculation.     

Coexistence of two sympatric and morphologically similar bat species Rhinolophus affinis and Rhinolophus pearsoni

Ecologists have long focused on the coexistence of sympatric species. Here, we investigated two horseshoe bat species, Rhinolophus affinis and Rhinolophus pearsoni inhabited in the same cave, for their foraging strategies, niche differentiation, prey selection, and their coexistence status. These two species of horseshoe bats were different in the dominant frequency of their echolocation calls, but similar in their morphology. We found evidence for prey selectivity although there was a high degree of overlap in prey categories and sizes. R. affinis and R. pearsoni foraged on 16 and 7 categories insects, respectively, with Pyralidae, Geometridae, Melolonthidae dominating their diets. The degree of trophic niche overlap was 0.69. Pairwise comparisons suggested that there was no obvious differentiation in prey categories and size. However, high prey availability in the environment （Simpson diversity index = 0.79 and Margalef richness index = 4.12） contributed to their coexistence by dampening the interspecific competition. Since there are one or more mechanisms facil- itating species coexistence in a community, our results suggest that the spatial niche differentiation in foraging rnicrohabitats and in foraging habitats at landscape scale may promote the coexistence of the two bat species. However, additional field data are needed to confirm this speculation.     

New Sivapithecus humeri from Pakistan and the relationship of Sivapithecus and Pongo

New humeri of two species of the Miocene hominoid Sivapithecus are described from near Chinji in Pakistan from between approximately 9 and 11 Myr ago. Sivapithecus, a middle and late Miocene hominoid from Turkey and Indo-Pakistan, is overall unlike any living hominoid, although facial-palatal similarities to the extant orangoutan, Pongo, have been used to support a hypothesis of close relationship. Living hominoids have postcranial similarities assumed to be shared derived, among them features of the proximal humerus. However, the new Sivapithecus proximal humeri differ from those of living hominoids, supporting an alternative hypothesis in which Sivapithecus and Pongo are not closely related. It is not clear how to choose between these incompatible hypotheses.     

Coexistence of two sympatric and morphologically similar bat species Rhinolophus affinis and Rhinolophus pearsoni

Ecologists have long focused on the coexistence of sympatric species. Here, we investigated two horseshoe bat species, Rhinolophus affinis and R. pearsoni inhabited in the same cave, for their foraging strategies, niche differentiation, prey selection, and their coexistence status. These two species of horseshoe bats were different in the dominant frequency of their echolocation calls but similar in their morphology. We found evidence for prey selectivity although there was a high degree of overlap in prey categories and sizes. R. affinis and R. pearsoni foraged on 16 and 7 categories insects, respectively, with Pyralidae, Geometridae, Melolonthidae dominating their diets. The degree of trophic niche overlap was 0.69. Pairwise comparisons suggested that there was no obvious differentiation in prey categories and size. However, high prey availability in the environment (Simpson diversity index = 0.79 and Margalef richness index ＝ 4.12) contributed to their coexistence by dampening the interspecific competition. Since there are one or more mechanisms facilitating species coexistence in a community, our results suggest that the spatial niche differentiation in foraging microhabitats and in foraging habitats at landscape scale may promote the coexistence of the two bat species. However, additional field data are needed to confirm this speculation.     

一类含对流项的反应扩散捕食模型正解的存在性

本文研究了一个含对流项的反应扩散捕食模型正解的存在性,该模型描述了两物种间的捕食关系及捕食者选择在远离高密度食饵区捕猎的倾向. 基于模型正解的先验估计, 本文利用特征值理论和齐次化理论获得了模型正解关于两物种增长率的不存在性,然后利用分岔理论获得了模型正解在某些参数条件下的存在性.     

An obligate brood parasite trapped in the intraspecific arms race of its hosts

Reciprocal selection pressures often lead to close and adaptive matching of traits in coevolved species. A failure of one species to match the evolutionary trajectories of another is often attributed to evolutionary lags or to differing selection pressures across a geographic mosaic. Here we show that mismatches in adaptation of interacting species--an obligate brood parasitic duck and each of its two main hosts--are best explained by the evolutionary dynamics within the host species. Rejection of the brood parasite's eggs was common by both hosts, despite a lack of detectable cost of parasitism to the hosts. Egg rejection markedly reduced parasite fitness, but egg mimicry experiments revealed no phenotypic natural selection for more mimetic parasitic eggs. These paradoxical results were resolved by the discovery of intraspecific brood parasitism and conspecific egg rejection within the hosts themselves. The apparent arms race between species seems instead to be an incidental by-product of within-species conflict, with little recourse for evolutionary response by the parasite.     

Determinants of establishment success in introduced birds.

A major component of human-induced global change is the deliberate or accidental translocation of species from their native ranges to alien environments, where they may cause substantial environmental and economic damage. Thus we need to understand why some introductions succeed while others fail. Successful introductions tend to be concentrated in certain regions, especially islands and the temperate zone, suggesting that species-rich mainland and tropical locations are harder to invade because of greater biotic resistance. However, this pattern could also reflect variation in the suitability of the abiotic environment at introduction locations for the species introduced, coupled with known confounding effects of nonrandom selection of species and locations for introduction. Here, we test these alternative hypotheses using a global data set of historical bird introductions, employing a statistical framework that accounts for differences among species and regions in terms of introduction success. By removing these confounding effects, we show that the pattern of avian introduction success is not consistent with the biotic resistance hypothesis. Instead, success depends on the suitability of the abiotic environment for the exotic species at the introduction site.     

Competition and phylogeny determine community structure in Müllerian co-mimics

Until recently, the study of negative and antagonistic interactions (for example, competition and predation) has dominated our understanding of community structure, maintenance and assembly. Nevertheless, a recent theoretical model suggests that positive interactions (for example, mutualisms) may counterbalance competition, facilitating long-term coexistence even among ecologically undifferentiated species. Müllerian mimics are mutualists that share the costs of predator education and are therefore ideally suited for the investigation of positive and negative interactions in community dynamics. The sole empirical test of this model in a Müllerian mimetic community supports the prediction that positive interactions outweigh the negative effects of spatial overlap (without quantifying resource acquisition). Understanding the role of trophic niche partitioning in facilitating the evolution and stability of Müllerian mimetic communities is now of critical importance, but has yet to be formally investigated. Here we show that resource partitioning and phylogeny determine community structure and outweigh the positive effects of Müllerian mimicry in a species-rich group of neotropical catfishes. From multiple, independent reproductively isolated allopatric communities displaying convergently evolved colour patterns, 92% consist of species that do not compete for resources. Significant differences in phylogenetically conserved traits (snout morphology and body size) were consistently linked to trait-specific resource acquisition. Thus, we report the first evidence, to our knowledge, that competition for trophic resources and phylogeny are pivotal factors in the stable evolution of Müllerian mimicry rings. More generally, our work demonstrates that competition for resources is likely to have a dominant role in the structuring of communities that are simultaneously subject to the effects of both positive and negative interactions.     

云南松的种群遗传与进化

本文从种群遗传学的角度研究了云南松（pinus yunnanensis)的遗传体制，种群内的多态现象，种群的遗传结构，演化潜力，生态小种和地理小种以及共交种等问题，得出的若干结论如下：（1）种内存在巨大的遗传变异，如在自然 群中表明的，几乎所有可见的形态性状都具有多态现象，这种巨大的遗传变异是自然选择进化的一个必要条件。（2）自然种群中，大多数个体的基因型很多座位都是杂合的，它们常表现出杂种优势，并且也可能表明增强了生理上发育上的体内平衡。（3）由于种群大，而且又分割成许多相对独立的亚群，各种进化要素都有能同时起作用，并已分化出生态种和地理小种，（4）云南松具有拟态种子，其起源和适应意义作了初步讨论。     

Effect of acoustic clutter on prey detection by bats.

Bats that capture animal prey from substrates often emit characteristic echolocation calls that are short-duration, frequency-modulated (FM) and broadband. Such calls seem to be suited to locating prey in uncluttered habitats, including flying prey, but may be less effective for finding prey among cluttered backgrounds because echoes reflecting from the substrate mask the acoustic signature of prey. Perhaps these call designs serve primarily for spatial orientation. Furthermore, it has been unclear whether the acoustic image conveyed by FM echoes enables fine texture discrimination, or whether gleaning bats that forage in echo-cluttering environments must locate prey by using other cues, such as prey-generated sounds. Here we show that two species of insectivorous gleaning bats perform badly when compelled to detect silent and immobile prey in clutter, but are very efficient at capturing noisy prey items among highly cluttered backgrounds, and both dead or live prey in uncluttered habitats. These findings suggest that the short, broadband FM echolocation calls associated with gleaning bats are not adapted to detecting prey in clutter.     

Pattern of nucleotide substitution at major histocompatibility complex class I loci reveals overdominant selection

The major histocompatibility complex (MHC) loci are known to be highly polymorphic in humans, mice and certain other mammals, with heterozygosity as high as 80-90% (ref. 1). Four different hypotheses have been proposed to explain this high degree of polymorphism: (1) a high mutation rate, (2) gene conversion or interlocus genetic exchange, (3) over dominant selection and (4) frequency-dependent selection. In an attempt to establish which of these hypotheses is correct, we examined the pattern of nucleotide substitution between polymorphic alleles in the region of the antigen recognition site (ARS) and other regions of human and mouse class I MHC genes. The results indicate that in ARS the rate of nonsynonymous (amino acid altering) substitution is significantly higher than that of synonymous substitution in both humans and mice, whereas in other regions the reverse is true. This observation, together with a theoretical study and other considerations, supports the hypothesis of overdominant selection (heterozygote advantage).